Why You Might See Horseshoe Crabs Swimming Belly Up

The horseshoe crab in the amazing YouTube clip captured at Aquaria KLCC in Malaysia appears to have gotten itself in a tricky situation! You normally see them scuttling around the ocean floor, but these horseshoe crabs sometimes flip onto their backs while swimming or during rough seas. Here’s why it happens. One is upside down, with its protective shell (carapace) dragging along the sand. Here, we will examine this seemingly bizarre behavior and explain why the horseshoe crab is acting this way.

Horseshoe Crab Physiology and Habitat

Despite their common name, horseshoe crabs are not true crabs. They are arachnids and are therefore related to spiders and scorpions. Unlike crabs, they have no antennae and no jaws but have spider-like appendages that they use for crushing and grasping food. They have existed for more than 400 million years; however, there are only four living species on Earth today. Horseshoe crabs are described as benthic animals, meaning they spend their time at the bottom of a body of water. Young horseshoe crabs stay in shallow water areas, but adults can go as deep as 650 feet. Here, they feed on slow-moving bottom-dwelling animals such as shellfish and marine worms.

How Do Horseshoe Crabs Move Around?

Horseshoe crabs use five of their six pairs of legs to walk or push themselves forward along the seafloor. To be more precise, the first four clawed legs lift them off the ground, and their pusher legs propel them forward. However, when they need to move quickly, they force water out of the gills under their body like a propulsion jet.

The crab in the clip, however, is clearly upside down and is ‘swimming’ using both its legs and the book gills on its underside. There are several occasions when you may find a horseshoe crab upside down. If sea conditions are rough, they can get flipped over by the ocean currents. Under these circumstances, it can be difficult for them to right themselves. It is even more challenging if they are injured. Young horseshoe crabs also swim upside down quite often. It seems to be easier for them to float in this position, and their gills work better for propulsion. Experts believe that swimming helps them spread to other areas (since they can move faster), and it may also help them get over barriers they cannot climb.

Getting the Right Way Up

An injured horseshoe crab can usually right itself. In deeper water, they simply rotate their bodies. In shallower water, they may need a little help from a vertical surface, as shown in the clip. They use their tail and legs to help themselves—then they scuttle away!